On the day of the Nativity [of our Lord] Dr. Martin Luther said many things about this remarkable festival which offers godly people the greatest pleasure and consolation, for the dear angels themselves preach and accompany their preaching with their singing.
“Those are true witnesses to the Messiah who declare, ‘Rejoice, be glad! Here you have the real Prince and a child of heaven and earth!’ They sing the best song: Glory be to God (that is, religion and faith), peace on earth (namely, in civil life), good will (that we feel right and are well pleased). It isn’t known, however, whether the angels were prophesying or expressing a wish, whether these words are to be understood indicatively(as a fact) or optatively(as a wish for us)…. (Luther’s Works, v. 54, p.248)